Before you buy a harness for a dog, make sure you have measured it correctly. It is done with a tape measure that must not slack while measuring. Try to make your dog stand straight. In order to measure the sizes of the harness you should take 3 main measurements:
Length from the top of shoulder to the bottom of the tail.
Chest girth in the widest part behind the forepaws. Add 2 cm for lean-body.
Neck girth should be taken at the location of the dog collar.
Learn more how to measure your dog to buy the right harness.